(a) Field of the Invention
This invention generally relates to a reinforced tubular structure. More specifically, this invention relates to a lightweight reinforced tubular structure and method of fabrication thereof, such structure having an outer metal tube and an inner composite tubular liner bonded to the tube.
(b) Description of the Prior Art
Metal tubular components of various shapes, i.e. round, rectangular, etc., such as drive shafts, tie rods, dead axles, cross members, and steering mechanisms, are normally required to convey torque from a power source to a means for converting this energy into useful work. For example, in the case of conventional automobiles, and other vehicles, the drive shaft conveys torque from the transmission to the differential where it is converted into the drive force for the rear wheels of the vehicle.
Conventionally, such tubular structures such as automotive drive shafts, are constructed of steel, or similar dense material, and have a substantial diameter and thickness in order to provide sufficient stiffness to meet the required torque and torsion requirements. The excessive weight of such shafts significantly increases the cost of producing and running the vehicle by increasing fuel consumption of the vehicle, reducing shaft critical vibration speed, and increasing cost of the shaft itself. Decreasing the thickness of the shaft to reduce weight has not been a satisfactory solution, because while such a shaft could carry torsion loads, longitudinal stiffness would not be sufficient to meet the drive shaft torque carrying and critical speed requirements.
The above considerations are even more significant in longer drive shafts such as those for long-bed trucks. Because of the length requirement for the drive shafts, two or more drive shafts are used because the weight/stiffness ratio of a single metal drive shaft would result in too low a critical speed. The multiple drive shafts are connected by a support bearing and frame structure. Use of the support bearing and frame structure increases the weight and cost of the overall drive shaft due to the extra parts required and labor for installation thereof.
To obviate the aforementioned difficulties, tubular structures, such as drive shafts, have been fabricated using composite materials. Typically, these materials are formed of a resinous matrix reinforced with layers of filamentary material, such as Kevlar, boron, or carbon fibers. However, such composite tubular members have not been entirely satisfactory. While light in weight and being able to provide shaft stiffness, composite tubular members have not been satisfactory with regard to carrying of torsion loads. In addition, composite material is subject to foreign object damage and environmental effects. In this regard, chipping in the surface of the composite material caused by flying objects can cause delamination and effect an imbalance of the shaft. Environmental effects such as those resulting from moisture, chemical solvents, and heat can also result in delamination by breakdown of the adhesive bonding of the laminate. Connecting of composite tubular structures to metal end members, such as for drive shafts, has also presented a problem. In addition, while users of vehicles (particularly trucks) appreciate weight reduction of the vehicle, there is resistance to visible non-metal substitutes for steel drive shafts (because of concern with foreign object damage and environmental effects on the shaft).